Methods of receiving in a forming mold a prescribed quantity of glass melt continuously flowing out of a nozzle, forming it into a sphere or similar shape, and employing the glass object obtained as a preform in the forming of molded optics or the like are known. Good preforms are manufactured not with a cutting blade, which causes the glass received in the forming mold to retain traces that could become defects in the press formed article, but by using the constriction produced in the flowing glass when the quantity of glass flowing out has reached a certain quantity to naturally sever the glass. The examples of Japanese Unexamined Patent Publication (KOKAI) Heisei No. 5-147949 (the Publication) describe such a method.
In the method disclosed in the Publication, the front end portion of the glass melt is received by the rim portion of the forming mold, and when the weight of the front end portion reaches a prescribed level, the constriction portion is used to separate the front end portion and slide it into the forming mold, with gas blown upward through gas outlets in the bottom of the forming mold being used to float the glass melt and form it into a sphere.
In the molding of preforms described in the Publication, numerous preforms are molded by a method in which forming molds are positioned at equal intervals about the rotational axis of a turntable undergoing indexed rotation, continuously flowing glass melt is received by successive forming molds and shaped, the shaped preforms are removed from the forming molds, and glass melt is received anew in the emptied forming molds.
The problem lies in that when the forming molds are moved, the gas blown upward from the forming molds sticks to the nozzle, with the nozzle and the glass flowing out of the nozzle ending up cooling. In the method of the Publication, this problem does not occur during receiving of the glass melt (referred to as casting) because the nozzle is not directly above the gas outlet of the forming mold. However, the gas being blown out of the gas outlets ends up being blown onto the nozzle when the forming mold is moved into the casting position and when the forming mold is moved out of the casting position.
In the forming of preforms, particularly in precision press forming such as the forming of molded optics, it is necessary to precisely control the weight of the preform and mold preforms without defects such as devitrification or striae. Viewed from this perspective, the blowing of the gas onto the nozzle even briefly is a substantial practical problem in that it results in a change of viscosity in the glass melt, thereby affecting the flow rate.
Accordingly, the present invention aims to solve the problem of gas from the gas outlet blowing onto the nozzle and changing the flow rate by altering the viscosity of the glass melt when the forming mold is moved into or out of casting position. The object of the present invention is to provide a method and device of manufacturing glass preforms with little variation in weight, and a method of employing the molded glass objects produced by this method as preforms in press molding to manufacture molded glass articles.